Eukaryotic cells contain genes in their nuclei as well as in certain organelles. The proper functioning of eukaryotic cells requires the close meshing of the expression of all genomes. The program of which this project is a part is directed at understanding how the multiple genomes relate to one another and how their expression is integrated. Plastids are organelles that can differentiate into several functional forms. Two projects are described. One is focused on identifying genes that are expressed differently in some cell types but not in other adjacent cells in a single organ, on learning whether genes of each expression class have common elements that distinguish them from genes of other expression classes, on recognizing mechanisms for limiting transcription of genes that are transcribed at elevated rates during one part of a developmental program and on identifying components of the transcriptional apparatus that may interact with these gene sequences. The second project has similar objectives. It is directed at understanding plastid genes that are expressed in the organism when it is illuminated but not when it is grown in darkness. The program is concerned with the molecular biology of gene expression in eukaryotic cells. This problem is of central importance in understanding biological processes in health and disease.